Disposable pump cassettes are frequently employed to infuse medicinal fluids into a patient. A pumping cassette may include a plastic housing having a front and a rear portion, between which an integral elastic membrane is encapsulated. One part of the housing has a number of ports through which actuators of a pump drive mechanism extend, interacting with the elastic membrane to control fluid flow through the cassette. A pump plunger on the drive unit presses against the membrane to reciprocatively pressurize liquid trapped in a pumping chamber formed between the membrane and the back of the housing. Similarly, actuator rods extend from the drive unit through ports in the housing, pressing against the membrane to interrupt fluid flow through valve passages formed in the back of the housing. A microprocessor in the pump drive controls the pump plunger and actuator rods to effect a desired rate of delivery of medicinal fluids to the patient, and in some units, is capable of selecting between a plurality of different sources by opening an appropriate selector valve in the cassette.
Selection of the source fluid and pumping rate or volume are normally determined by an operator programming the pump drive in response to a display prompt. Significant leakage through the valves in the pump cassette can create a potentially harmful variation from the programmed value in the quantity of medication actually delivered to a patient, or in the case of a leaking selector valve, may allow a medicinal fluid to enter the pump cassette when infusion of the fluid into the patient is not desired. Leakage of the valves or in other parts of the pump cassette, e.g., due to a poor seal between the elastic membrane and housing, is difficult or impossible to detect by visual inspection and may occur after the cassette was originally inspected for leaks during its manufacture. In view of the potential harm to the patient should significant leakage go undetected, there is clear justification for evaluating the leakage integrity of all valves and of the cassette assembly when it is first used to administer drugs, and perhaps at periodic intervals thereafter.
Apparatus and a method for detecting valve leakage in a pump cassette are disclosed in U.S. Pat. No. 4,657,490. A reciprocating plunger pressurizes and pumps fluid form a pumping chamber in the cassette described in this patent. To check for leakage, a stepping motor advances the plunger to elevate the pressure of liquid trapped in the pumping chamber between an inlet valve and an outlet valve. Attached to the plunger is a load cell that produces a signal indicative of the force required to pressurize the liquid trapped in the pumping chamber. If the signal produced by the load cell fails to indicate that an increase in force is required to elevate the pressure of the liquid, a system problem is indicated. Either the liquid supply is depleted, gravity feed of liquid is insufficient to fill the pumping chamber, or the inlet or outlet valve or both are leaking, allowing liquid to escape from the pumping chamber as the plunger is advanced.
Use of a load cell to detect multiple causes of system failure reduces the complexity of the device; however, this approach cannot determine which of the three possible problems has been detected. Moreover, in the disclosed method shown in the prior art, there is no provision for testing other valves in the cassette for leakage. The present invention provides a more effective apparatus to detect and identify valve leakage.